They come from Gambler's General Store in many cases. You can buy unopened packages of dice from many closed clubs there. If they run out of the one you are looking for, there will probably be another production run, so check back in a couple of weeks.

One of the reasons I don't have a lot of interest in dice collecting is you can't tell the genuine article from the reproductions unless you only collect those that are starting to crystalize from age. There's one such from Flamingo on eBay this week.